This invention relates to a clock pulse control system, and more particularly to a clock pulse control system for use in a microcomputer system having a central processing unit (CPU) and a plurality of input/output (I/O) units including an I/O unit to operate at a different speed from that of the CPU.
A microcomputer system compries a CPU including a data processing circuit and a memory device for storing a program or a variable data; a plurality of I/O units connected to CPU, a clock pulse generator for operating the I/O units and CPU, and a selector channel adapted to generate a signal for selecting one or a plurality of I/O units by a device control instruction from CPU.
In the above-mentioned microcomputer system clock pulses having a common freequency with respect to the CPU and I/O units are given from the clock pulse generator to effect an instruction or data transfer between the CPU and the I/O unit selected by the signal of the selector channel.
To the CPU and I/O units is given a frequency band of predetermined range over which they can be operated. A clock pulse having a common frequency with respect to the CPU and I/O units is selected from among their frequency band.
In a microcomputer system, the operating speed of the system including a central processing unit is governed by the frequency of a clock pulse utilized to operate various input/output units connected to the CPU.
For example, where an I/O unit in which that frequency range of a clock pulse as most suited for the operation of the I/O unit is different from that frequency range of the clock pulse as most fitted for the operation of the system, is connected to the above-mentioned system, it is impossible to connect said I/O unit to the microcomputer system when there is no common frequency that can operate said I/O unit and the system.
Where there exists a frequency which permits the system and I/O unit to be operated in a common range shared between the frequency range of the system and the frequency range of the I/O unit, it is possible to operate the system and I/O unit by a clock pulse having such a common frequency.
Where a clock pulse having a common freqeucny can be used to operate the microcomputer system there may occur a case wherein either one of said two types of the input/output units can not operate with an optimum frequency but operates with a frequency substantially different from the optimum frequency. Consequently, it is impossible to operate efficiently both types of the input/output units.
Moreover, when, as mentioned above, respective input/output units have different frequency bands of clock pulse it is necessary to select the clock frequency of the input/output unit having lower operation speed as the common clock frequency. For this reason, in order to make it possible to use a clock frequency in common for different type of input/output units it is necessary to operate an input/output unit at a speed lower than its intrinsic high operating speed. More particularly, any input/output unit has an intrinsic operating speed at which it can operate most efficiently.
For example, the above-mentioned microcomputer system is applied to an electronic cash register and the second input/output unit is formed of a display unit using an indication type multidigit nixie tube. Clock pulses used to operate the CPU of the aforesaid microcomputer system are essentially required to have a frequency ranging between 200 and 300 KHz. In practical application, there are used clock pulses having a frequency of 250 KHz selected from said frequency range. The clock pulse for actuating the display unit have a frequency ranging from 200 KHz to 1 MHz. If, however, said display unit is not operated by a clock pulse having a higher frequency than 400 KHz when dynamically displaying data by itself, then the resultant display pattern will flicker too much to be easily recognized by the human eye.
Accordingly, a disadvantage occurred that it is necessary to make the operating speed of the entire system equal to that of an input/output unit whose operating speed should be limited.
Accordingly, it is an object of this invention to provide a novel clock pulse control system for use in a microcomputer system including an input/output unit operative at a frequency band partially common to, and partially different from, the operating frequency band of a central processing unit.